This invention relates to optical waveguide devices, and more particularly to passive optical waveguide devices, as well as integrated optical circuits including passive optical waveguide devices.
In the integrated circuit industry, there is a continuing effort to increase device speed and increase device densities. Optical systems and technologies promise to deliver increasing speed and circuit packing density in the future. Optical waveguides typically include optical waveguide devices to provide optical functionality. Such optical waveguide devices can perform a variety of optical functions in integrated optical waveguide circuits such as optical signal transmission and attenuation.
In one aspect, optical waveguide devices include a variety of passive optical waveguide devices and/or a plurality of active optical waveguide devices. For example, certain gratings, lenses, filters, photonic crystals, and the like can be fabricated as passive optical waveguide devices. Similarly, active optical waveguide devices may function as filters, gratings, lenses, deflectors, switches, transmitters, receivers, and the like. Availability of a variety of passive and active optical waveguide devices and/or electronic devices provides a desired range of functionality. The availability of these devices is useful in making optical waveguide circuits simpler to design and fabricate.
A passive optical device does not change its function over a period of time excluding device degradations. A large variety of passive optical devices that include, e.g., optical fibers, slab optical waveguides, or thin film optical waveguides, may provide many optical functions. As such, the output or optical functionality of passive optical waveguide devices cannot be tuned or controlled. Additionally, passive active devices cannot be actuated (i.e., or turned on and off) depending on the present use of a region of an optical waveguide.
Many active optical waveguide devices such as modulators, filters, certain lenses, and certain gratings are precisely tunable. Tunability of certain active optical waveguide devices is important in making them more functional and competitive with present electronic circuits and devices.
Silicon-on-Insulator (SOI) and CMOS represent two technologies that have undergone a considerable amount of research and development relating to electronic devices and circuits. SOI technology can also integrate optical devices and circuits. It would be desirable to provide active optical waveguide device functionality and/or passive optical waveguide device functionality based largely on the CMOS devices and technology as well as manufacturing methods that allow for simultaneous fabrication of optically active and passive waveguide elements.
One embodiment of prior-art optical waveguide device is an arrayed waveguide grating (AWG) as shown in FIG. 2. The AWG 400 includes an input coupler 402, a plurality of arrayed waveguides 404, and an output coupler 406. The AWG 400 can be configured as a wavelength-division demultiplexer (if light signals travel from the left to the right in FIG. 2) or a wavelength-division multiplexer (if light signals travel from the right to the left in FIG. 2). In the AWG 400, each arrayed waveguide 404 has a different length between the input coupler 402 and the output coupler 406. The difference in length between each one of the different arrayed waveguides 404 corresponds to an optical phase shift of m2xcfx80, where m is an integer for the central design wavelength of the light that is applied to the AWG 400. Since each arrayed waveguide 404 has a different length, the light passing through the longer arrayed waveguides arrives at the output coupler 406 later than the light passing through the shorter arrayed waveguides.
AWGs 400, however, are difficult and expensive to produce. Each arrayed waveguide 404 is measured and formed separately. The operation of the AWG 400 requires that the different arrayed waveguides 404 differ in length by a distance equal to an m2xcfx80 optical phase shift for the central design wavelength that the AWG is designed to multiplex/demultiplex. The cross-sectional area and the material of each arrayed waveguide 404 of the AWG 400 is constant to maintain the effective mode index (or the propagation constant xcex2) of the different arrayed waveguides 404, and therefore provide a uniform velocity of light traveling through the different arrayed waveguides. As such, in present designs, each arrayed waveguide 404 of the AWG 400: a) has precisely calculated and measured lengths; b) has the same precisely produced and measured cross-sectional areas; c) has different lengths, such that the difference between the successive lengths, xcex94l is such that xcex2 xcex94l=m2xcfx80; and d) is smoothly-curved through a gradual radius of curvature to reduce bending losses of light flowing through the arrayed waveguide 404. Due to these requirements, the AWG 400 is challenging to design and fabricate since it is difficult to ensure the precise relative lengths of each one of the arrayed waveguides 404. Both the precision requirements and fabrication tolerances place extreme requirements on the manufacturing process. These waveguides traditionally use different indices of glass to make the core and the cladding. Silicon is used in the fabrication process but does not participate in the optical function. A 6xe2x80x3 Si wafer may be able to accommodate 5-50 AWGs 400 depending on the design requirements and the available index contrast between the core and the cladding, which is generally of the order of a few percent. The waveguides in AWGs are designed to be polarization independent so that both the polarizations of the input light are more or less treated equally. Considerable time and human effort is therefore necessary to produce precise AWGs 400.
It would therefore be desirable to fabricate passive optical waveguide devices (such as AWGs) using standard CMOS fabrication techniques which, when combined with active optical functions such as a modulator on the same substrate, could form the basis of a WDM system on a chip. It would also be desirable to fabricate such passive optical waveguide devices as AWGs and interferometers in a manner that the lengths and shapes of the arrayed waveguides are simple to accurately calculate, measure, and produce. Furthermore, it would be desired to apply active optical waveguide devices as tuning devices associated with optical circuits including passive optical waveguide devices, wherein much of the fabrication errors inherent in passive optical waveguide devices or device degradation over time can be dynamically tuned out by tuning the associated active optical waveguide devices.
One aspect of the invention relates to a passive optical waveguide device deposited on a wafer. The wafer includes an insulator layer and an upper semiconductor layer formed at least in part from silicon. The upper silicon layer forms at least part of an optical waveguide, such as a slab waveguide. The passive optical waveguide device includes an optical waveguide, a gate oxide, and a polysilicon layer (i.e., a layer formed at least in part from polysilicon). In some embodiments, the optical waveguide is formed within the upper semiconductor layer, a gate oxide layer that is deposited above the upper semiconductor layer, and a polysilicon layer that is deposited above the gate oxide layer. The polysilicon layer projects a region of static effective mode index within the optical waveguide. The region of static effective mode index has a different effective mode index than the optical waveguide outside of the region of static effective mode index. The region of static effective mode index has a depth extending within the optical waveguide. A value and a position of the effective mode index within the region of static effective mode index remains substantially unchanged over time. The region of static effective mode index applies a substantially unchanging optical function to light travelling through the region of static effective mode index over the lifetime of the passive optical waveguide device.
As explained below, the terms xe2x80x9cgate oxidexe2x80x9d or xe2x80x9cgate oxide layerxe2x80x9d as used herein refer to the type of oxides (or other electrically insulating materials including glass) that are typically used to form a gate regardless of whether the material is used functionally to form all or part of a gate. Each region of static/altered effective mode index described herein is due to the presence of polysilicon deposited on the xe2x80x9cgate oxidexe2x80x9d layer, and controlled (at least in part) by controlling the shape or dimensions of the polysilicon. The polysilicon acts to guide light though one or more layers of a wafer (similar to a rib waveguide) and, depending on the width and height of the polysilicon, acts to create a region with a different effective mode index or having a different propagation constant, as compared to remaining regions on the wafer. Various xe2x80x9cphotonic guidesxe2x80x9d may be created simply by the presence of polysilicon deposited on the gate oxide. Optionally, a layer below the gate oxide layer (e.g., an upper silicon layer of an SOI substrate) may also be etched to create total reflection boundaries that also serve to define the xe2x80x9cphotonic guide.xe2x80x9d By positioning different xe2x80x9cphotonic guidesxe2x80x9d (or polysilicon portions) in appropriate geometric relationships on a substrate as described herein, many useful passive and/or active optical devices may be fabricated using well understood manufacturing steps of electronic device manufacturing. Different portions of the xe2x80x9cphotonic guidesxe2x80x9d may be made active by construction of appropriate electrodes for diode or transistor action and local, variable effective mode index created, as described below. Exemplary passive complex functions formed using the xe2x80x9cphotonic guidesxe2x80x9d described herein include AWG""s for separation and combining of different colors of light in the waveguide, interferometers, lenses, and gratings.
One aspect of the invention relates to an integrated optical circuit comprising an optical waveguide and an evanescent coupler. The optical waveguide is located on a wafer. The optical waveguide is formed from an upper semiconductor layer of the wafer, a gate oxide layer deposited on the upper semiconductor layer, and a polysilicon layer deposited on the gate oxide layer. The evanescent coupling region is formed at least in part from a gap portion that optically couples light to the upper semiconductor layer of the optical waveguide using the evanescent coupling region. Light can be coupled from outside of the passive optical waveguide device via the evanescent coupling region into the optical waveguide. Alternatively, light can be coupled from the optical waveguide through the evanescent coupling region out of the passive optical waveguide device. The polysilicon layer projects a region of static effective mode index within the optical waveguide, wherein the region of static effective mode index has a different effective mode index than the optical waveguide outside of the region of static effective mode index. A value and a position of the effective mode index within the region of static effective mode index remains substantially unchanged over time and applies a substantially unchanging optical function to light travelling through the region of static effective mode index within the optical waveguide over the lifetime of the passive optical waveguide device.
One aspect of the invention relates to an optical waveguide device that controls the transmission of light through an optical waveguide. The optical waveguide device comprises an active optical waveguide device and a passive optical waveguide device. The active optical waveguide device is formed at least in part on a semiconductor layer and includes an electrode portion. A region of altered effective mode index is created by the active optical waveguide device. An effective mode index of the region of altered effective mode index within the optical waveguide is controlled by application of an electric voltage to the electrode portion in a manner that alters a free carrier density of the region of altered effective mode index. Changing the electric voltage to the electrode portion changes the effective mode index in the region of altered effective mode index. The passive optical waveguide device is formed at least in part from a polysilicon layer deposited on the semiconductor layer. An effective mode index of a region of static effective mode index within the optical waveguide is created by the polysilicon layer of the passive optical waveguide device. The polysilicon layer has a shape and a height. The effective mode index of the region of static effective mode index is related to the shape of the polysilicon layer and the height of the polysilicon layer. A value and a position of the effective mode index within the region of static effective mode index remains substantially unchanged over time and applies a substantially unchanging optical function to light travelling through the region of static effective mode index over the lifetime of the passive optical waveguide device. The optical waveguide forms at least a part of both the active optical waveguide device and the passive optical waveguide device. The optical waveguide couples the active optical waveguide device and the passive optical waveguide device, and the optical waveguide is formed at least in part using the semiconductor layer. In one aspect, the active optical waveguide device can be configured to provide electronic transistor action.
One aspect of the present invention relates to an interferometer comprising at least one optical waveguide, a first passive optical waveguide segment, and a second passive optical waveguide segment. The at least one optical waveguide includes at least one gate oxide layer deposited on a semiconductor layer of a wafer and a polysilicon layer deposited on the at least one gate oxide layer. The first passive optical waveguide segment includes a first portion of the polysilicon layer. The first portion projects a first region of static effective mode index within the at least one optical waveguide. The first region of static effective mode index has a different effective mode index than the at least one optical waveguide outside of the first region of static effective mode index. A value and a position of the effective mode index within the first region of static effective mode index of the first passive optical waveguide segment remains substantially unchanged over time. The first region of static effective mode index therefore applies a substantially unchanging optical function to light travelling through the first region of static effective mode index within the at least one optical waveguide over the lifetime of the first passive optical waveguide segment. The second passive optical waveguide segment includes a second portion of the polysilicon layer. The second portion projects a second region of static effective mode index within the at least one optical waveguide. The second region of static effective mode index has a different effective mode index than the at least one optical waveguide outside of the second region of static effective mode index. A value and a position of the effective mode index within the second region of static effective mode index of the second passive optical waveguide segment remains substantially unchanged over time and applies a substantially unchanging optical function to light travelling through the second region of static effective mode index within the at least one optical waveguide over the lifetime of the second passive optical waveguide segment. A length of the first passive optical waveguide segment equals a length of the second passive optical waveguide segment. The first and second passive optical waveguide segments are coupled to each other and together form at least in part the optical waveguide. The first and second passive optical waveguide segments and the optical waveguide are each formed at least in part from the semiconductor layer. The first region of static effective mode index has a different effective mode index than the second region of static effective mode index. In one embodiment, the difference in effective mode between the first and the second region of static effective mode index is at least partially provided by a difference in cross-sectional areas respectively between the first portion of the polysilicon layer and the second portion of the polysilicon layer. In another embodiment, the difference in effective mode between the first and the second region of static effective mode index is at least partially provided by a difference in axial lengths respectively between the first portion of the polysilicon layer and the second portion of the polysilicon layer.
One aspect of the present invention relates to an arrayed waveguide grating (AWG) deposited on a wafer that includes an upper semiconductor layer comprising a first port, a plurality of second ports, a gate oxide layer, a polysilicon layer, and a plurality of arrayed waveguides. The gate oxide layer is deposited above the upper semiconductor layer. The polysilicon layer is deposited above the gate oxide layer. The plurality of arrayed waveguides extend between the first port and each one of the plurality of second ports. Each one of the plurality of arrayed waveguides are at least partially formed by the upper semiconductor layer, the polysilicon layer, and the gate oxide layer. Each one of the arrayed waveguides is associated with a portion of the polysilicon layer. Each portion of the polysilicon layer has a different cross-sectional area, wherein each of the arrayed waveguides has a different effective mode index. A value and a position of the effective mode index associated with each of the respective arrayed waveguides remains substantially unchanged over time and applies a substantially unchanging optical function to light travelling through the respective arrayed waveguide over the lifetime of the respective arrayed waveguide. In one embodiment, the different effective mode indexes in each of the respective arrayed waveguides is provided by a difference in cross sectional area of the polysilicon layer associated with each one of the plurality of arrayed waveguides. In another embodiment, the different effective mode indexes in each of the respective arrayed waveguides is provided by a difference in axial length of the polysilicon layer associated with each one of the plurality of arrayed waveguides.
One embodiment of the present invention relates to an optical waveguide device that controls the transmission of light through an optical waveguide. The optical waveguide device includes a first passive optical waveguide device and a second passive optical waveguide device. The first passive optical waveguide device is etched, at least in part, in a semiconductor layer of a wafer. A value and a position of an effective mode index within the first passive optical waveguide device remains substantially unchanged over time and applies a substantially unchanging optical function to light travelling through the first passive optical waveguide device over the lifetime of the first passive optical waveguide device. The second passive optical waveguide device is formed at least in part from a polysilicon layer deposited above an unetched portion of the semiconductor layer. The effective mode index of a region of static effective mode index within the optical waveguide is created by the polysilicon layer of the second passive optical waveguide device. The effective mode index of the region of static effective mode index is related to a shape of the polysilicon layer and a height of the polysilicon layer. A value and a position of the effective mode index within the region of static effective mode index remains substantially unchanged over time, and applies a substantially unchanging optical function to light travelling through the region of static effective mode index over the lifetime of the second passive optical waveguide device. The optical waveguide forms at least a part of both the first passive optical waveguide device and the second passive optical waveguide device. The optical waveguide couples the first passive optical waveguide device and the second passive optical waveguide device, and the optical waveguide is formed at least in part using the semiconductor layer.
One aspect of the present invention relates to a device that provides for the transmission of light through a first optical waveguide and a second optical waveguide. The device includes a semiconductor layer and a polysilicon coupler. The semiconductor layer includes at least one etched portion between first and second unetched portions. The first optical waveguide includes the first unetched portion and a first total internal reflection (TIR) boundary between the first unetched portion and the at least one etched portion. The second optical waveguide includes the second unetched portion and a second TIR boundary between the at least one unetched portion and the second etched portion. The polysilicon coupler at least partially overlaps the etched portion of the semiconductor layer. The polysilicon coupler optically couples the first optical waveguide and the second optical waveguide, wherein light can flow from the first optical waveguide via the polysilicon coupler portion to the second optical waveguide.
One aspect of the present invention relates to a passive optical waveguide device, comprising a silicon layer of a Silicon-on-Insulator (SOI) wafer, a gate oxide layer that is often fabricated on glass, and the polysilicon layer. The gate oxide layer is commonly used during the fabrication of electronic transistors. The polysilicon layer is often used during the fabrication of electronic transistors. The polysilicon layer is often used to form a portion of a gate electrode used in Field Effect Transistors (FET). The glass layer is deposited on the silicon layer, and the polysilicon layer is deposited on the glass layer. By controlling the width and the height of the polysilicon layer the effective mode index or the propagation constant xcex2 is controlled to provide a rib or ridge optical waveguide. Many structures that perform a variety of optical functions can be constructed by adjusting the polysilicon parameters (e.g., shape, dimension, height, etc.). Furthermore, optical waveguide devices such as AWGs, can be constructed in an existing CMOS fab, using cost effective techniques and processes. Certain passive optical waveguide devices that can be constructed using the techniques described herein include, e.g.,: rectangular AWGs, lenses and lens arrays, adiabatic tapers, and Bragg structures. Many embodiments of passive optical waveguide devices can be constructed in thin SOI by etching the silicon layer. Examples of passive optical waveguide devices that are formed by etching the silicon layer in thin SOI include mirrors, mirror arrays, Echelle gratings, MMI, adiabatic tapers, coupled waveguides, and focusing Echelle devices.